There has been much recent interest in the development of various methods of isotopic separation or enrichment. Although much of this effort has been directed to the separation of uranium isotopes, pure or enriched isotopes of other elements are also desirable, for example, as tracer materials for medical research and diagnosis, biological research and environmental studies.
Multiphoton dissociation of molecules using intense infrared lasers has been the object of extensive investigation during the past few years (see Mukamel et al, J. Chem. Phys. 65, 5204 (1976) and Dever et al, J. Am. Chem. Soc. 98, 5055 (1976) as well as the references cited therein). An important application of this technique is to the separation of isotopically labeled molecules, as reviewed in several references (Mukamel et al and Dever et al supra; Walther "Atomic and Molecular Spectroscopy with Lasers", Topics in Applied Physics, Vol. 2, Laser Spectroscopy, (Walther, Ed.) Springer-Verlog, 1976; Laser Spectroscopy, Proceedings of the Second International Conference, Megeve June 23-27, 1975 (Haroche et al, Ed.), Springer-Verlog, 1975; Letokhov et al, Sov. J. Quant. Electr. 6, 129,259 (1976); and Aldridge III et al "Experimental and Theoretical Studies of Laser Isotope Separation", Physics of Quantum Electronics, Vol. 4, Laser Photochemistry, Tunable Lasers, and Other Topics, Jacobs et al, Ed., Addison-Wesley, 1976). Enrichment in the isotopes of H, B, C, Si, Cl, S and Os has been reported.
V.S. Letokhov, Physics Today, May 1977, pages 23-31, provides a review of the art including a tabular recitation of successful laser isotope separations including the separation of C.sup.12 and C.sup.13 by multiphoton dissociation of CCl.sub.4. This reference also describes a pulsed TEA CO.sub.2 -He-N.sub.2 laser which has been employed in laser isotope separations.
Another recent survey article of interest is R. N. Zare "Laser Separation of Isotopes", Scientific American, Feb. 1977, Vol. 236, No. 2, pages 86-98.
Multiphoton dissociation of CCl.sub.3 F and CF.sub.3 Cl has been reported by Dever et al supra. Dever et al used a focused CO.sub.2 laser to obtain up to 1.6% conversion of the parent molecule per flash at about 60 torr of pressure. No investigation of the isotopic selectivity was reported.
Lyman et al, J. Appl. Phys. 47 595 (1976) have described enriching carbon-13 by mutliphoton dissociation of CF.sub.2 Cl.sub.2 (Freon-12). The .sup.13 C/.sup.12 C ratio of the starting material was increased by a factor of 1.65 by selectively dissociating .sup.12 CF.sub.2 Cl.sub.2.
The invention described herein using CF.sub.3 I offers several advantages over the processes shown in the above two references. CF.sub.3 I can be dissociated at relatively low intensities. An unfocused TEA CO.sub.2 laser gives sufficient power so that a measurable fraction of starting material can be dissociated in fewer than 100 shots at one torr in a reasonable cell volume. In addition, very high isotope separation factors may be achieved in CF.sub.3 I. In excess of 15% of the molecules in the beam can be dissociated per laser pulse at high intensities, and enrichment factors of nearly 600 have been obtained.
The interaction of low intensity CO.sub.2 laser radiation with CF.sub.3 I has been reported by Jones et al, J. Mol. Spectr. 58 125 (1975) and Petersen et al, Opt. Commun. 17, 259 (1976).
Photochemical isotopic enrichment techniques are based on two main phenomena. First, there is the fact that the wavelengths of spectral lines absorbed by a molecule depend somewhat on the isotopes present in the molecule. Second, the rate of a chemical reaction is sometimes influenced by the state of excitation of the participating molecules. In order for photochemical isotopic enrichment to be possible with a given starting material, several conditions must be satisfied. First of all, the effect of isotopic content of the starting material on the wavelengths of one or more of its spectral lines must be large enough so that one type of isotope-containing molecule could be preferentially excited by absorbing laser radiation which would not excite the other type of isotope-containing molecules. Secondly, a laser is needed whose radiation happens to match in wavelength one of the isotope-dependent lines, or a laser that can be tuned to such a wavelength, and the spectral width of the laser radiation must be narrow enough to excite molecules containing one of the isotopes and not the others. Thirdly, the recombination of the fragments selectively dissociated by the laser to form the original starting compound must be retarded or prevented. Fourthly, transfer of excitation from one molecule to another by collisions, and "scrambling" of isotopes through collisions of reaction products with other species must be negligible. Both these latter factors contribute to the overall selectivity of the process.